The invention relates to a method and apparatus for obtaining improved signal relative to noise without loss of ion collection efficiency for use in mass spectrometry, including LC/MS (liquid chromatography/mass spectrometry), especially as regards the technique of generating analyte ions known as Atmospheric Pressure Chemical Ionization (APCI).
Liquid chromatography and mass spectrometry have proven powerful analytical tools in identifying molecular components of our world. Liquid chromatography is a fundamental separation technique. Mass spectrometry is a means of identifying xe2x80x9cseparatedxe2x80x9d components according to their characteristic xe2x80x9cweightxe2x80x9d or mass-to-charge ratio. The liquid effluent from LC is prepared for ionization and analysis using any of a number of techniques. A common technique, electrospray, involves spraying the sample into fine droplets.
Early systems for electrospray LC/MS utilized flow splitters that divided the HPLC (high performance liquid chromatography) column effluent. As a result of the effluent splitting, only a small portion, typically 5-50 micro liters per minute, was introduced into the xe2x80x9cspray chamberxe2x80x9d. The bulk of the column effluent did not enter the spray chamber but went directly to a waste or fraction collector. Because electrospray/mass spectrometry (ES/MS) generally provides a concentration sensitive detector, it was not necessary to analyze the entire column effluent flow to obtain sensitive results. Results obtained by splitting are comparable in sensitivity to those obtained by introduction of the entire column effluent flow into the spray chamber (assuming equal charging and sampling efficiencies).
Such low flow rates enabled generation of an electrosprayed aerosol solely through the manipulation of electrostatic forces. However, the use of flow splitters gained a bad reputation due to potential plugging problems and poor reproducibility.
Newer electrospray systems generate a charged or ionized aerosol through the combination of electrostatic forces and some form of assisted nebulization. Nebulization is the process of breaking a stream of liquid into fine droplets. Nebulization may be xe2x80x9cassistedxe2x80x9d by a number of means, including but not limited to pneumatic, ultrasonic or thermal assists. The assisted nebulization generates an aerosol from the HPLC column effluent, while electric fields induce a charge on the aerosol droplets. The charged aerosol undergoes an ion evaporation process whereby desolvated analyte ions are produced. Ideally, only the desolvated ions enter the mass spectrometer for analysis.
A challenge in any assisted nebulizer system, is designing the vacuum system leading to the mass spectrometer such that desolvated ions enter, but relatively large solvated droplets present in the electrosprayed aerosol are prevented from entering. Several design approaches arc currently in use, but none has solved all the challenges. None of the assisted nebulization methods currently practiced provide reliable sensitivity along with robust instrumentation.
In conventional electrospray/nebulization mass spectrometry systems, the electrosprayed aerosol exiting from the nebulizer is sprayed directly towards the sampling orifice or other entry into the vacuum system. That is, the electrosprayed aerosol exiting from the nebulizer and entry into the vacuum system are located along a common central axis, with the nebulizer effluent pointing directly at the entry into the vacuum system and with the nebulizer being considered to be located at an angle of zero (0) degrees relative to the common central axis.
One previous approach directed at improving performance adjusts the aerosol to spray xe2x80x9coff-axisxe2x80x9d. That is, the aerosol is sprayed xe2x80x9coff-axisxe2x80x9d at an angle of as much as 45 degrees with respect to the central axis of the sampling orifice. In addition, a counter current gas is passed around the sampling orifice to blow the solvated droplets away from the orifice. The gas velocities typically used generate a plume of small droplets. Optimal performance appears to be limited to a flow rate of 200 microliters per minute or lower.
In another system, an aerosol is generated pneumatically and aimed directly at the entrance of a heated capillary tube; the heated capillary exits into the vacuum system. Instead of desolvated ions entering the capillary, large charged droplets are drawn into the capillary and the droplets are desolvated while in transit. The evaporation process takes place in the capillary as well. Exiting the capillary in a supersonic jet of vapor, the analyte ions are subsequently focused, mass analyzed and detected.
This system has several disadvantages and limitations, including sample degradation, re-clustering, and loss of sensitivity. Sensitive samples are degraded due to the heat. In the supersonic jet expansion exiting the capillary, the desolvated ions and vapor may recondense, resulting in solvent clusters and background signals. While these clusters may be re-dissociated by collisionally induced processes, this may interfere in identification of structural characteristics of the analyte samples. The large amount of solvent vapor, ions and droplets exiting the capillary require that the detector be arranged substantially off-axis with respect to the capillary to avoid noise due to neutral droplets striking the detector. Removing the large volume of solvent entering the vacuum system requires higher capacity pumps.
Still another system generates the electrosprayed aerosol ultrasonically, uses a counter current drying gas, and most typically operates with the electrosprayed aerosol directed at the sampling capillary. Several serious disadvantages plague this configuration. The optimal performance is effectively limited to less than five hundred (500) microliters per minute. Adequate handling of the aqueous mobile phase is problematic. Furthermore, the apparatus is complex and prone to mechanical and electronic failures.
In another commonly used system, a pneumatic nebulizer is used at substantially higher inlet pressures (as compared with other systems). This results in a highly collimated and directed electrosprayed aerosol. This aerosol is aimed off axis to the side of the orifice and at the nozzle cap. Although this works competitively, there is still some noise which is probably due to stray droplets. The aerosol exiting the nebulizer has to be aimed carefully to minimize noise while maintaining signal intensity; repeated and tedious adjustments are often required.
While the techniques are varied with respect to the type of nebulization assist, techniques can be broadly characterized along the lines of what process is used for accomplishing ionization of the analyte. Atmospheric Pressure Ionization-Electrospray (API-ES or ES herein) and Atmospheric Pressure Chemical Ionization (APCI) differ in the ionization mechanism. Each technique is suited to complementary classes of molecular species.
The techniques are, in practice, complementary owing to different strengths and weaknesses. Briefly, APT-ES is generally concentration dependent (that is to say, higher concentration equals better performance), and performs well in the analysis of moderately to highly polar molecules. It works well for large, biological molecules and pharmaceuticals, especially molecules that ionize in solution and exhibit multiple charging. API-ES also performs well for small molecules, provided the molecule is fairly polar. Low flow rates enhance performance. APCI, on the other hand, performs with less dependence on concentration and performs better on smaller non-polar to moderately polar molecules. Higher flow rates enhance performance.
At the most fundamental level, APCI involves the conversion of the mobile phase and analyte from the liquid to the gas phase and then the ionization of the mobile phase and analyte molecules. APCI is a soft ionization technique that yields charged molecular ions and adduct ions. APCI, as implemented in the hardware described herein, actually includes several distinct ionization processes, with the relative influence of each process dependent on the chemistry of the mobile phase and the analyte. What is desired is an assisted nebulization LC/MS configuration for APCI that operates in a complementary range of flow rates as does API-ES. What is further needed and wanted from the practitioner""s point of view is a mass spectrometry apparatus easily and interchangeably configurable for operation in either API-ES or APCI mode with increased sensitivity in both operating modalities. What is further desired is robust instrumentation that provides sensitive results without constant calibrating or other process interruptive maintenance procedures.
In one embodiment the invention relates to an apparatus for converting a liquid solute sample into vaporized and ionized molecules comprising:
a first passageway having a center axis, an orifice for accepting a liquid solute sample, an interior chamber within which the liquid solute sample is converted into vaporized molecules, and an exit for discharging the vaporized molecules;
a charged point voltage source having the point arranged adjacent to the first passageway exit which ionizes the vaporized molecules into ionized molecules;
an electrically conductive housing connected to a second voltage source and having an opening arranged adjacent to the first passageway exit wherein the ionized molecules formed by the point charge voltage source are interposed between the point charge voltage source and the housing;
a second passageway arranged within the housing adjacent to the opening and connected to a third voltage source, the second passageway having a center axis, an orifice for receiving ionized molecules and an exit, wherein the center axis of the second passageway is arranged in transverse relation to the center axis of the first passageway such that the ionized molecules move laterally through the opening in the housing and thereafter pass into the second passageway under the influence of electrostatic attraction forces generated by the second and third voltage sources.
In another embodiment the invention relates to an apparatus for converting a solute sample into ionized molecules, comprising:
a first passageway having a center axis, an orifice for accepting a solute sample, an interior chamber within which the solute sample is vaporized, and an exit for discharging the vaporized molecules;
a charged-point voltage source having the point arranged adjacent to the first passageway exit for ionizing the vaporized molecules;
a second passageway connected to a voltage source and arranged a distance from the exit of the first passageway, the second passageway having an entrance having a center axis, an orifice for receiving the ionized molecules from the first passageway, and an exit, wherein the center axis of the second passageway is arranged in transverse relation to the center axis of the first passageway such that the ionized molecules move laterally into the orifice of the second passageway under the influence of electrostatic attraction forces generated by an electric field; and
a housing adjacent to the second passageway wherein a voltage source is connected to the housing.
In another embodiment the invention relates to an apparatus for converting a liquid solute sample into ionized molecules, comprising:
(a) a first passage way having a center axis and an exit;
(b) a charged-point voltage source arranged adjacent to said exit of the first passageway;
(c) a second passageway having a center axis;
(d) a housing adjacent to the second passageway wherein a voltage source is connected to the housing;
(e) at least one additional voltage source connected to at least one of the passageways;
wherein the first passageway is capable of converting the liquid solute sample into vaporized molecules;
wherein the charged-point voltage source is capable of converting the vaporized molecules into ionized molecules;
wherein the additional voltage source results in a difference in potential thereby creating an electric field sufficient to move ionized molecules into the second passageway; and
wherein the center axis of the first passageway is positioned transverse to the center axis of the second passageway at an angle of from about 75 degrees to about 105 degrees.
In another embodiment the invention relates to an apparatus for converting a solute ample into ionized molecules, comprising:
a first passageway having a center axis, an orifice for accepting a solute sample, an interior chamber within which the solute sample is vaporized, and an exit for discharging the vaporized molecules,
a charged-point voltage source having the point arranged adjacent to the first passageway exit for ionizing the vaporized molecules;
a second passageway arranged a distance from the exit of the first passageway, the second passageway having an entrance having a center axis, an orifice for receiving the ionized molecules from the first passageway, and an exit, wherein the center axis of the second passageway is arranged in transverse relation to the center axis of the first passageway such that the ionized molecules move laterally into the orifice of the second passageway under the influence of electrostatic attraction forces generated by an electric field; and
an electrically conductive element connected to a voltage source, wherein the element is arranged adjacent to the exit of the first passageway and wherein vaporized molecules exiting the first passageway is interposed between the element and the entrance of the second passageway.
The invention provides the capability of ionizing effluent from conventional high performance liquid chromatography (HPLC) at flow rates of greater than one (1) ml/minute without flow splitting. The invention provides that ionization may be accomplished in a variety of manners, including atmospheric pressure chemical ionization (APCI) as well as atmospheric pressure ionization electrospray (API-ES).
As applied to API-ES, the invention further provides that desolvated ions be separated from comparatively large volumes of vaporized aerosol from the column effluent, and then, while keeping out as much of the aerosol as possible, introducing the desolvated ions into the vacuum system for mass detection and analysis. The invention provides the capability of separating desolvated ions from the large volumes of vapor and directing the desolvated ions from the ionization chamber (typically operating at atmospheric pressure) to the mass spectrometer (MS) (operating at 10xe2x88x926 to 10xe2x88x924 torr). The inventive separation capability preserves instrument sensitivity because the maximum amount of analyte (in the form of desolvated ions) is introduced into vacuum system to be mass analyzed and detected. Furthermore, the inventive sensitivity is preserved without overwhelming the vacuum system with large volumes of liquid droplets or vapor.
Orthogonal ion sampling according to the present invention allows more efficient enrichment of the analyte by spraying the charged droplets in the electrosprayed aerosol past a sampling orifice, while directing the solvent vapor and solvated droplets in the electrosprayed aerosol away from the ion sampling orifice such that they do not enter the vacuum system.
As applied to APCI, the invention further provides that ions be separated from comparatively large volumes of vaporized column effluent, and then, while keeping out as much of the vapor as possible, introducing the ions into the vacuum system for mass detection and analysis. The invention provides the capability of separating desolvated ions from the large volumes of vapor and directing the desolvated ions from the ionization chamber (typically operating at atmospheric pressure) to the mass spectrometer (MS) (operating at 10xe2x88x926 to 10xe2x88x924 torr). The inventive separation capability preserves instrument sensitivity because the maximum amount of analyte (in the form of ions) is introduced into the vacuum system to be mass analyzed and detected, but incomplete solvent-to-vapor phase change in the heater does not appear as noise, in contrast to the situation with the straight-on configurations of the prior art. Furthermore, the inventive sensitivity is preserved without overwhelming the vacuum system with large volumes of liquid droplets or vapor and residual liquid-phase solvent.
The noise level in an apparatus configured according to the present invention is reduced by as much as five fold over current systems, resulting in increased signal relative to noise, and hence achieving greater sensitivity. Performance is simplified and the system is more robust because optimization of the position of the first passageway, gas flow and voltages show less sensitivity to small changes. The simplified performance and reduced need for optimization also result in a system less dependent upon flow rate and mobile phase conditions. The reduced need for optimization extends to changing mobile phase flow rates and proportions. Practically speaking, this means that an apparatus configured to employ the inventive system can be run under a variety of conditions without adjustment.